We had the same problem with DS1 - we did the change at 24 months because I was pregnant and wanted it all worked out a fair bit before the baby came. The first night he wouldn't got to sleep till midnight, then gradually got down to a reasonable bedtime, i had to lay on the bed with him until he was asleep, then after about a week, put a chair next to the bed and sat on that - it probably took 6 weeks all up to get to the point where I could take him to bed and say good night and he would go to sleep himself. It is a huge adjustment to them, did he choose a doona cover, etc, etc. I guess a reward chart may work...it just takes time and being consistent....sorry, I'm not much help.